Switching contactor

ABSTRACT

A switching electrical power contactor having a bi-blade type switch, has ferrous plates attached to the blades to increase the current carrying capacity and reduce the resistance of the switch.

CROSS REFERENCE TO RELATED APPLICATIONS

This non-provisional patent application claims priority under 35 U.S.C.§119(a) from Patent Application No. GB1200331.5 filed in United Kingdomon Jan. 9, 2012.

FIELD OF THE INVENTION

This invention relates to an electrical power switching contactor and inparticular, to a single-pole or two-pole contactor capable of switchingcurrents of more than 80 amps at mains voltage.

This invention also relates to the types of high-current switchingcontactors employed in modern electricity meters, so-called “smartmeters”, for performing a pre-payment or safety-disconnect function atnormal domestic supply (mains) voltages, e.g. 100 V AC to 240 V AC. Ithas a particular application to electrical contactors having a bi-bladecontact arrangement as described in U.S. Pat. No. 7,833,034.

BACKGROUND OF THE INVENTION

Many contactors of this type are capable of switching nominal current atsay 100 Amps or 200 Amps, for a large number of switching load cycles,satisfactorily, the switching being done by suitable silver-alloycontacts containing certain additives, which prevent welding. Theswitching blades are configured to be easily actuated for the switchingfunction, with minimal self heating at the nominal currents concerned.

Most meter specifications not only stipulate satisfactoryNominal-current Endurance switching—without the contacts welding—butalso demand that at moderate short-circuit fault conditions they mustalso not weld, and must open on the next actuator-driven pulse. At muchhigher related “dead-short” conditions the switch contacts may weld, butmust remain intact, not explode or emit any dangerous molten materialduring the “dead-short” duration, until protective fuses rupture, orcircuit breakers drop-out and disconnect the mains supply to the load,safely. This shorting duration may be for a maximum of 6 cycles of themains supply.

U.S. Pat. No. 7,833,034 introduced the basic configuration of the“bi-blade” switch comprising a pair of parallel movable spring-copperarms or blades, of a particular thickness, width and active length, witha small defined gap there between. The blades' fixed ends are terminatedtogether by rivets, screws, or semi-shears, to a moving-blade-carrierterminal, with movable contacts attached on the inner faces of the freeends, which close naturally on fixed contacts attached to the otherfixed-blade-carrier terminal of the switch.

In the basic embodiment, the contactor uses a bi-blade switchconstruction, in which the switch has a pair of movable arms (also knownas blades), which are strip-punched and pre-formed so that they close onthe fixed contacts with a defined “contact-pressure” force—for achievinga relatively low switch resistance—and the open ends are formedoutwardly with a sloping portion. The arms extend parallel to each otherand separated by a small gap so that under high current situations thecurrents through the arms create forces of magnetic attraction urgingthe arms towards each other and increasing the force applied to thefixed contacts disposed between the distal ends of the arms. This forceof attraction offsets the repulsive force urging the contacts apart, andis also due to the high current passing through the contacts. Thisarrangement is shown in FIGS. 1 to 3. FIGS. 1 & 2 show a single-polecontactor 10 with the cover removed to show the workings. FIG. 3 is aschematic view of the arms 30 of one switch. Each arm has a strip ofspring copper having a first end 34 attached to a first terminal 24,known as the movable terminal as it is connected to the movable arms. Asecond terminal 22, known as the fixed terminal has fixed contacts 23.The distal end 36 of each arm is fitted with a movable contact 25. Eacharm 30 has a sloping section or portion 38 to create an offset betweenthe ends of the arms such that the fixed contacts can be accommodatedbetween the movable contacts. The two arms extend parallel to each otherexcept at the sloping portion. The movable contacts are arranged toalign with the fixed contacts and in the relaxed state of the arms, themovable contacts bear against the fixed contacts with a predeterminedcontact force. The arms are able to move or flex within the plane of thedrawing about the connection to the first terminal. A rib 39 is formedin the arms to stiffen the arms against excessive flexing.

The basic parallel “bi-blade” configuration, as used in a 100 Ampnominal current contactor, creates dynamic magnetic blade forces inexcess of the contact repulsion forces during short-circuit faults. Theblade geometries and contacts were optimised to avoid welding at thespecified operating conditions. This basic 100 Amp switch uses 4contacts; two movable and two fixed, with 50 Amps in each parallelblade. This basic arrangement was not capable of withstanding muchhigher nominal and short-circuit currents, as the blade geometries andcurrent-sharing parameters limited the balancing of the blade forces andparticularly the greater contact repulsion forces, resulting in muchlessened endurance life, and serious contact welding issues duringhigher short-circuit faults.

U.S. Pat. No. 7,833,034 also introduced the divided blade concept,allowing a 200 Amp nominal current contactor able to balance the dynamicmagnetic blade forces and contact repulsion forces during short-circuitfaults, the geometries and contacts being optimised to avoid welding atthe specified conditions.

To evenly share the current sharing—and to balance the repulsive contactforces and blade magnetic attraction forces—each adjacent parallel“bi-blade” was sub-divided into longitudinal half-blades, with a movablecontact at each of their free ends, mating with respective fixedcontacts, thus constituting 4 half-blades in parallel with 8 contactsper switch, or 16 in total for the 2-pole, two-phase disconnectcontactor. This lower current sharing in each half-blade significantlyreduces the contact repulsion forces.

Thus at 200 Amps, each half-blade will be carrying only 50 Amps,reducing the burden per half-blade when switched, minimising selfheating, and avoiding welding at the higher nominal and short-circuitcurrents. Importantly, all half-blade currents flow in the samedirection, thus maximising the magnetic attraction forces betweenhalf-blades in the working gap, especially at high current, to keep thecontacts tightly closed.

The existing 100 Amp switch designs using simple parallel spring-copper“bi-blades” are very limited by the geometries and gap between, eachblade in the “bi-blade” set being capable of generating certain magneticattraction forces at high shared current, one with-respect-to the other,balanced and acting against the contact repulsion forces—both beingproportional to the square of the current—in order to ensure that thecontacts remain closed during short-circuit faults. It is very difficultto get this balanced ratio of forces exactly right for a particularconfiguration. Hence the divided blade version was optimised for use at200 Amps, but used longer blades and 16 contacts in total.

The divided bi-blade configuration provided a good solution for the 200Amp contactor but at a price as the silver contacts are expensive andthe divided blades take up space. There is also a market want for the100 Amp and 200 Amp contactors to be made smaller to save space. Thusthere is a desire to reconfigure the simpler, basic parallel “bi-blade”100 Amp switch geometry and configuration, so it was capable to operateat the higher 200 Amps nominal current with a greater short-circuitcapability, in full compliance with various National requirements suchas the ANSI C12.1 meter-disconnect specification.

Certain embodiments of the present invention provide a smaller, simpler,cost-reduced switch, using a new “bi-blade” switch arrangement, whichnot only uses less copper blade material, but requires only 8 switchingcontacts per 2-pole contactor instead of the current 16 required in thepresent design for a contactor rated at 200 Amps nominal current.Silver-alloy contacts represent a significant proportion of allhigh-current contactor cost breakdowns, so a reduction in the number ofcontacts required for a particular switching function is a majorcost-saving benefit. Teachings from the improvements to the 200 Ampcontactor can be applied to contactors rated at 100 Amps or less, toreduce its size.

SUMMARY OF THE INVENTION

Accordingly, in one aspect thereof, the present invention provides anelectrical contactor comprising: a first terminal connected to a pair offixed contacts on opposite faces of a fixed conductive member; a secondterminal; a pair of movable arms of electrically conductive materialconnected to the second terminal, and carrying movable contacts at anend remote from the connection to the second terminal, the movable armsbeing arranged in aligned opposition to each other such that theirremote ends are on either side of the fixed conductive member, with themovable contacts aligned with the fixed contacts, and are separated by apredetermined gap over a major portion of their length; and ferrousplates attached on the outer faces of the movable arms, wherein thearrangement of the fixed member and movable arms being such that whenthe contacts are closed, current flowing through the movable arms andthe ferrous plates produces induced magnetic-field attraction forcesbetween the movable arms that urges the movable arms towards each other,thereby increasing the force pressing the movable contacts against thefixed contacts.

Preferably, the movable arms are preformed and preloaded so as to biasthem towards each other to engage the fixed contacts with a presetcontact pressure keeping the contacts normally closed in the absence ofa force separating the movable arms.

Preferably, the ferrous plates are attached to the movable arms alongtheir formed length, whereby when the contacts are closed, highercurrent flowing through the movable arms induces magnetic fields in theferrous plates, generating a magnetic force of attraction urging thecontacts closed.

Preferably, an actuating arrangement is provided that includes a wedgeshaped member disposed between inner inclined surfaces of the movablearms, arranged to separate the movable arms so as to open the contacts,the wedge shaped member being movable from a first position in which itseparates the movable arms, to a second position where it allows thearms to move freely towards each other.

Preferably, the actuating arrangement comprises a movable member that,in a first position is not engaged with the movable arms, to allow thewedge shaped member to separate the movable arms and in a secondposition engages with the movable arms to press the movable contactsagainst the fixed contacts.

Preferably, the movable member comprises pegs that press against outerinclined surfaces of the movable arms in the second position.

Preferably, the outer inclined surfaces of the movable arms are separateflexible tangs integral with the movable arms, which deflect inwardly,to enhance the contact closure and reduce bounce when engaged by themovable member in the second position.

Preferably, the actuating arrangement comprises an electromagneticactuator coupled to the wedge shaped member and the movable member, theelectromagnetic actuator effecting movement of the wedge shaped memberand the movable member, between the first and second positions.

Preferably, the wedge and the movable member are part of a lifter fixedto the electromagnetic actuator.

Preferably, an actuating arrangement is provided, the actuatingarrangement including an electromagnetic actuator, the electromagneticactuator being released or de-latched to cause the movable contacts toengage the fixed contacts.

Preferably, the electromagnetic actuator is a magnet-latching solenoid.

Preferably, each movable arm is arranged to carry a substantially equalportion of the total current through the contactor.

Preferably, each movable arm comprises a plurality of longitudinalsections, each provided with a movable contact at the remote end andarranged to engage with a corresponding fixed contact, the current flowin the arms being substantially equally divided between the sectionsthereof.

Preferably, the first and second terminals are made of brass.

Preferably, the contactor is a two pole contactor having a pair of firstand second terminals, a pair of fixed conductive members and two pairsof movable arms.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to figures of the accompanying drawings. Inthe figures, identical structures, elements or parts that appear in morethan one figure are generally labeled with a same reference numeral inall the figures in which they appear. Dimensions of components andfeatures shown in the figures are generally chosen for convenience andclarity of presentation and are not necessarily shown to scale. Thefigures are listed below.

FIG. 1 is a plan view of a single pole contactor having bi-blade movablearms, according to the prior art, the contactor is shown with a coverremoved;

FIG. 2 is a perspective view of the contactor of FIG. 1;

FIG. 3 is a schematic view of a pair of bi-blade movable arms accordingto the prior art;

FIG. 4 is a schematic view similar to FIG. 3, of a pair of bi-blademovable arms according to the preferred embodiment of the presentinvention, engaging contacts of a fixed member;

FIG. 4 a is a plan view of a variation of the movable arms of FIG. 4;

FIG. 5 is a plan view of a two-pole contactor incorporating the movablearms of FIG. 4, with a cover removed;

FIG. 6 is a schematic view, similar to FIG. 4, of a pair of bi-blademovable arms according to a second embodiment of the present invention,shown in the open position;

FIG. 7 is a schematic view, of the pair of bi-blade movable arms of FIG.6, shown in the closed position;

FIG. 8 is a schematic isometric view of the movable arms of FIG. 5 andthe associated fixed member and terminals;

FIG. 9 is a schematic plan view of a two-pole contactor according to athird embodiment of the present invention;

FIG. 10 is an enlarged partial view of the contactor of FIG. 9, showingthe contacts of one pole in the fully open position;

FIG. 11 is a view similar to FIG. 10, showing the contacts in thepartially open position;

FIG. 12 is a view similar to FIG. 10, showing the contacts in the fullyclosed position;

FIG. 13 is a side view of a prior art meter enclosure;

FIG. 14 is a plan view of the meter enclosure of FIG. 14;

FIG. 15 is a side view of a meter enclosure according to the presentinvention;

FIG. 16 is a plan view of the meter enclosure of FIG. 15;

FIG. 17 is a schematic view of a wall box fitted with a disconnect meteraccording to the present invention; and

FIG. 18 is a schematic view of a 2-pole contactor having lead/lagswitches.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Four important improvement concepts (the improvements) will now bedescribed to illustrate the present invention. Each improvement will bediscussed with reference to one or more preferred embodiments offered byway of example to describe the invention. While each concept can becombined with the teachings of the other concepts, certain concepts canbe applied individually to prior art contactors of differentconstruction.

FIG. 4 is a schematic view of a pair of bi-blade movable arms 30 of anelectrical contactor, according to a preferred embodiment of the presentinvention. Each arm is similar to the prior art arms of FIG. 3 exceptthat the stiffing ribs 39 are replaced by ferrous plates, in the form ofsteel laminations 40, intimately attached to the outer surface of thearm. The steel lamination 40 extends over a majority of the length ofthe arm 30 and preferably extends over the sloped portion 38 and thedistal end 36 of the arm. In FIG. 4, the fixed terminal 22 and the fixedcontacts 23 are shown disposed between the movable contacts 25 with thearms 30 in the relaxed state such that the contacts are engaged, knownas the closed position. As before, the two arms 30 face each otheracross a small gap 33 for a majority of their length. The steellaminations allow the contact arms to be shorter for the same currentrating and also reduces the resistance of the switch. The steellaminations 40 are fixed to the arms 30 by riveting, preferably usingupset rivets 41 formed in the steel lamination and passing through holesin the arms.

This design allows the construction of a smaller, cost-reduced switch,with shorter, narrower spring-copper “bi-blades”, which would have alower nominal resistance and self heating, but which is also capable ofgenerating much larger magnetic attraction forces, to overcome theinevitably larger contact Repulsion forces at the greater sharedcurrents, with using fewer contacts.

With the standard, longer parallel copper “bi-blade” geometry, there isa defined magnetic attraction force between them at high sharedshort-circuit fault current, the strong individual magnetic fields beingin close proximity to each other, across the gap, augmenting each other,creating some deflection (inwardly) in both, and closing the related gapat the same time. If the short-circuit fault current is very high—as forexample during AC peaks—there is a danger that the blades may deflecttoo far, touch and possibly re-bound the contacts off, which willmomentarily Open the switch and destroy the “bi-blade” effect, withpotentially catastrophic explosive consequences.

FIG. 4 a illustrates a variation of the blades shown in FIG. 4. Whilethe steel enhanced bi-blade construction is designed to avoid usingdivided blades, for contactors with a very high current rating, over 200Amps or for very compact contactors, a steel enhanced bi-blade switcharrangement may be useful, especially if the number of longitudinalsections can be reduced by using the steel laminations. Hence, in FIG. 4a is an example of a steel enhanced, divided, bi-blade switch of acontactor. The switch has a pair of bi-blade arms 30 extending from amovable terminal 24 to which they are riveted (only one visible), to afixed terminal 22 having fixed contacts 23 opposing movable contacts 25fixed to the distal ends of the arms. Each arm is divided into aplurality of longitudinal sections (two shown) by a slot 43 extendingfrom the distal end towards the fixed end. Each longitudinal section hasa steel lamination 40 fixed to an outer surface, preferably by use of anupset rivet 41.

FIG. 5 illustrates a two-pole contactor 10 with a cover removed. Thecontactor has two switch sets 12, one on either side of a solenoid 16. Alifter 18 is fixed to a plunger of the solenoid and carries a wedge 50and two pegs 52 for each switch. The wedge is disposed between the armsand arranged to separate the arms when driven into the gap 33. The twopegs 52 are disposed on opposite sides of the pair of arms in the regionof the sloping portion 38. In the closed position as shown in FIG. 5,the pegs press against the outer surface of the sloping portion 38 ofthe arms, either directly or indirectly via the steel laminations, tourge the contacts to the closed position. When the solenoid moves thelifter to the open position, to the left as shown, the pegs disengagethe arms allowing the contacts to open as the wedge enters into the gap33 moving the distal ends of the arms apart opening the contacts. InFIG. 5, the contacts are obscured by the lifter 18, however opening andclosing of the contacts can be seen in FIGS. 6 & 7.

The solenoid 16 may be a self latching solenoid, preferably a magneticself latching solenoid which is pulse operated and spring biased to theclosed position. Thus in operation, the solenoid is pulsed to changestate, to latch in the open position or de-latch to the closed position.This saves energy as the solenoid is only momentarily energised tochange positions.

The shorter, narrower steel-enhanced “bi-blades” give the advantage thatthe switch nominal resistance is typically halved, while the magneticattraction forces between the movable arms are increased by at least afactor of five, as compared with the standard longer blades of the priorart.

The plug-in switch terminals or “stabs” for the standard 2-pole metercontactor, are normally tooled from 2.38 mm thick copper sheet or strip,for plugging-into the meter base sprung jaws. These copper tooled shapesgenerate considerable scrap loss. Since the steel-enhanced switchresistance is typically halved, it is possible to replace these copperterminals with brass terminals of the same thickness, achieving afurther cost saving of approximately 40%, due to the price differencebetween copper and brass. FIG. 5 illustrates a 2-pole plug-in metercontactor incorporating the shorter, narrower steel-enhanced“bi-blades”.

The 2-pole contactor has a symmetric layout of the two steel-enhancedswitches with the centrally-placed solenoid 16, driving a lifter 18attached to the solenoid plunger, having two wedges 50 for opening theblade sets. The terminal “stabs” 22, 24, enable the 2-pole contactor tobe plugged into the meter socket. By making the terminal stabs out ofbrass instead of copper, the cost of the contactor is further reduced.The solenoid is preferably of a long narrow construction, disposedbetween the two sets of blades, to allow the contactor to have arelatively small width, allowing the contactor to fit between the sprungjaws of the meter socket so that the standard wall box and meterconfiguration can be used.

In the 2-pole contactor shown in FIG. 5, with shorter spring-copper“bi-blades”, the presence of the stiffer steel laminations 40 attachedintimately to the copper arms 30 has removed the flexibility seen in thestandard blade design, which readily deflected inwardly under highshort-circuit fault conditions, giving some contact wiping which reducedmelt-pool tack-welding.

There is a concern that under high short-circuit fault conditions,stiffer arms such as the steel-enhanced bi-blades described above, mayvibrate and bounce off briefly under the massive blade attraction andcontact repulsion forces being balanced in the strong magnetic fields.Similarly, during nominal current switching, there is a concern that therigid blades could generate some unwanted contact bounce, potentiallycausing tack welds, worsening endurance life and contact delamination.

In order to eradicate these concerns, the contact or distal ends 36 ofthe arms 30 of the bi-blades are formed with a flexible tang 44 formedat one side as shown in FIGS. 6 to 8. As illustrated in FIGS. 5 and 6,the plunger of the solenoid 16 is attached to a lifter 18 with wedgeshaped extensions (wedges 50) which are placed between the offset distalends 36 of the blade pairs, so when the solenoid is driven, the bladesand contacts are opened via the wedge being moved into the gap 33between the arms and pressing against the inner blade faces of thesloping portion 38.

The lifter 18 also has pairs of “pegs” 52 which sit astride the outersides of the sloping blade faces. The pegs 52 are spaced from the arms30 when the lifter 18 is in the open position with the wedge 50 holdingthe arms apart. When the lifter is in the closed position, in which thewedge is disengaged from the arms, allowing the arms to close on thecontacts, thus closing the switch, the pegs 52 engage with and deflectthe tangs 44 inwardly, clamping the contacts gently so to preventbounce. Also, during high “carrying” short-circuit and “dead-short”fault conditions, any vibration due to the massive blade attraction andcontact repulsion forces being balanced, the peg 52 and tang 44 clampingreaction prevents bounce and spurious contact opening.

The tangs 44 are formed by making a longitudinal slit 46 in the distalend 36 of each arm, extending through the sloping portion 38 of theblade face. The tang does not contact the fixed contact and thus carriesno current. While the tang is shown extending to the end of the arm, asthe pegs only contact the sloping surface, the tang may be suitablymodified and adjusted to provide a desired level of additional contactpressure. The tang is not covered by the steel plate 40.

The flexible tang concept, while shown as part of the steel enhancedbi-blade construction, could be applied to simple bi-blade switches toenhance the contact pressure and thus reduce normal contact resistanceand improve resistance to contact bounce during contact closing.

In contactors described above, which use multi contacts (up to 16 intotal) for even current sharing at Nominal current or high short-circuitfault levels, it is important that the contacts used have adequate“top-lay” silver-alloy thickness, in order to withstand the arduouscurrent “switching” and “carrying” duties involved. Typical top-laythickness of an 8 mm diameter bi-metal contact is in the range 0.6 to1.0 mm, which equates to considerable cost, especially when 16 contactsare used in a 200 Amp, 2-pole contactor as used in prior art designsutilising a divided bi-blade construction.

One method of reducing the total silver-alloy cost is to control thetop-lay thickness in some contacts of each switch, by introducing aspecial switching concept referred to as “lead/lag”, which lends itselfvery well to the way the bi-blade arms are actually adjusted, set up andactuated during the pulse-driven switching function. This is even moreimportant in the shorter-blade, steel-enhanced switch proposed above,which only uses 8 contacts instead of 16. The contacts will be sized tosuit the Endurance life requirements.

With the “lead/lag” principle, as illustrated in FIGS. 9 to 13, chosenblades 30 and contacts 23, 24 in each set are adjusted and set up insuch a way that during closing of the contacts a defined but criticaltime delay is introduced between the contacts that first closes (the“lead” contacts 60) taking the brunt of the switching load current, andthe delayed contacts (the “lag” contacts 62) which closes a fractionlater in time. This always ensures that the lag contacts only carry loadcurrent, keeping it relatively clean and hardly eroded. Thus the lagcontacts 62 can have a much thinner top-lay silver-alloy thickness ascompared with the lead contacts.

On the other hand, the lead contacts 60 taking the brunt of theswitching load current (especially if the load is inductive) requires athicker top-lay than the lag contacts, to enhance endurance life andreduce contact-delamination. Thus when the blade adjustment, set up andpulse-drive is optimised for lead/lag, it is possible to makeconsiderable savings with the rationalised contacts as described.

It is possible, for example, to optimise a lead/lag contact set forrelatively thick top-lay on the switching lead contacts, and muchthinner top-lay on the carrying lag contact, making a considerablereduction in the silver-alloy content. Also the carrying lag contactsmay be smaller in diameter.

In a simple arrangement, the wedge 50 which opens the arms 30 of thebi-blade switch, may be set slightly offset such that the wedge does notclose the contacts or move the arms evenly. In particular, the wedge 50will move one arm 30 slightly ahead of the other arm causing one arm,the lead arm, to close the switch (movable contact engages the fixedcontact) slightly before the other arm, the lag arm, closes. FIG. 9illustrates the switching mechanism of the contactor 10. FIGS. 10 to 12are partial views which illustrate one set of switch contacts 23, 25,moving from the open position to the partially closed position and tothe closed position, on an enlarged scale. In FIG. 10, the contacts areopen with the wedge 50 holding the arms 30 apart, representing an openswitch. In FIG. 11, the wedge 50 has moved to a position intermediatethe open and close positions. At this position, one set of contacts, thelead set 60 have already made contact and thus the switch is closed.However, the other set of contacts, the lag set 62 are still held apart,thus no current can flow through the lag contacts 62. In FIG. 12, thewedge 50 has moved to the close position, releasing both arms 30allowing both sets of contacts, the lead contacts 60, and the lagcontacts 62, to close thus sharing the current flow through the switch.

In a 2-pole contactor, each switch may have a lead/lag contactarrangement as described above. Alternatively, as the two switches areeffectively in series with the load between the supply terminals, oneswitch may be designated as the switching switch and the other switch asthe carrying switch. In this case the carrying switch closes slightlybefore the switching switch so that it closes under a no currentcondition and the switching switch closes under full load conditions.Thus in terms of timing, the lead and lag roles are reversed but asbefore one set of contacts can be of lower current rating or using lessexpensive material, saving costs in the manufacture of the contactor. Inthis arrangement of 2-pole contactor, again the timing of the switchingoperation can be arranged by suitable positioning of the wedges whichseparate the arms, such that on release, one arm or one switch willclose before the other.

FIG. 18 is a schematic diagram of a 2-pole contactor with lead contactsset at different switches. The contactor 10 has a first switch 12 and asecond switch 12′. The first switch has a first terminal 22 carrying afixed contact 23, a second terminal 24 connected to a first movable arm30 carrying a movable contact 25 at an end remote from the connection tothe second terminal. The fixed contact 23 and the movable contact 25form a first switch pair of contacts 60. The second switch 12′ issimilarly constructed. The second switch has a third terminal 22′carrying a fixed contact 23′, a fourth terminal 24′ connected to asecond movable arm 30′ carrying a movable contact 25′ at an end remotefrom the connection to the fourth terminal. The fixed contact 23′ andthe movable contact 25′ form a second switch pair of contacts 62. Asolenoid 16 moves a lifter 18 between first and second positions. Afirst wedge 50 integral with the lifter moves the first arm 30 to openand close the first switch. A second wedge 50′ integral with the liftermoves the second arm 30′ to open and close the second switch. The wedgesare arranged, preferably by being offset, such that when the contactorcloses, that is going from an open state to a closed state, the firstswitch pair of contacts 60 close after the second switch pair ofcontacts close. That is, there is a delay in the closing of the firstswitch compared with the second switch. In this configuration thecontacts of the second switch take on the role of the lead contacts andhandle the switching load while the contacts 23, 25 of the first switch12 handle only carrying or load current and thus can be smaller. Thecontactor is shown with each switch having two arms but the conceptworks with switches having one or more arms.

There is a distinct cost advantage of incorporating a well adjusted andset up “bi-blade” set with “lead/lag” contacts as described above. Ifnot properly pulse-driven, even at nominal current, some lead contactscan tack weld during operational life, since with the erosion thatoccurs, some points on the switched silver-alloy surface can becomesilver-rich, which promotes more tack-welding randomly. This isespecially a problem if the pulse-drive is not strong enough to breakthe tack-welds that occur with switching bounce. Also depending on whenthis might happen through operational life, a tack-weld could occurduring a moderate short-circuit fault for the same reasons.

One arrangement to improve this tack weld problem is to use a silveralloy top-lay which is tungsten rich. In particular, a special silveralloy top-lay with tungsten-oxide additive inclusions in the silvermatrix, particularly for the lead switching contact. Addition oftungsten-oxide additive in the matrix has several important effects andadvantages:

1) it creates a more homogeneous “top-lay” structure, puddling theeroding surface more evenly, but not creating as much silver-rich areas,prone to tack welding,

2) it raises the general melt-pool temperature at the switching point,which discourages tack-welding, and

3) because the tungsten-oxide additive is a fair proportion of the total“top-lay” silver mass, for a given thickness, there is also a small costadvantage.

All the improvements described above can be used to create a smaller,cost-reduced, meter-disconnect contactor, which would normally bemounted inside a meter casing. This improved design is smaller than allthe existing meter-disconnect contactors, enabling it to be mounted notonly inside the meter casing conventionally, but also to be movedoutside of the meter envelope interface, either still attached to theunder-side of the meter base enclosure, or integrated and nestledbetween and within the sprung jaws of the meter terminal block of thewall-box. The sprung jaws are the terminals of the meter socket thatallow the mains meter to be simply plugged into the terminal block foreasy installation and replacement. As such the sprung jaws are arrangedaccording to a fixed conventional layout to allow compatibility betweenbrands and models.

The schematic diagrams of FIGS. 13 & 14 show a typical plug-in meterarrangement with the existing, larger disconnect-contactor 10 mountedinside the meter casing 70, notionally plugged into sprung jaws of ameter socket in a “wall-box” for safely connecting the meter via it'scopper terminal stabs to the supply and load cables mounted in the rearof the wall-box.

The existing larger meter-disconnect contactor mounted inside theplug-in meter casing as shown in FIG. 13 is too large to be mounted andattached below the meter-base molding, as the meter “stabs” 74 centerswould not be compatible with the sprung jaw centers in the wall-box.

To fit between the stabs, the meter-disconnect contactor would have tobe narrower, similar to the improved steel-enhanced contactor describedabove, for normal stab plugability of the meter into the wall-box sprungjaws, as shown in the schematic diagrams of FIGS. 15 & 16.

The smaller meter-disconnect contactor 10 able to be produced using theimprovements described above, is able to be mounted completely outsidethe meter enclosure 74, either on the back of the meter enclosurebetween the meter stabs as shown in FIGS. 15 & 16 or between the sprungjaws of the meter socket of the typical wall-box, as shown in FIG. 17,actually switching the sprung jaw connection.

In FIGS. 15 & 16, the contactor 10 is directed mounted to the back ofthe meter enclosure 74 between the terminal stabs 74 of the meter.Actually, two terminals of the contactor will be connected to two of thestabs. The meter enclosure 74 has four legs 76 which are disposed closeto respective stabs but outside of the space defined by the stabs. Thelegs 76 provide some protection for the stabs during transport and wheninstalled the legs sit against the wall box or the meter socket toensure correct positioning of the meter.

The 2-pole contactor of FIG. 17 is similar to the contactor shown inFIG. 5 and described hereinbefore. The contactor 10 has a symmetriclayout with two switches 12 having steel-enhanced blade sets, and acentrally-placed solenoid 16 driving a lifter 18 for opening the bladesets. The solenoid 16 is preferably of a long narrow construction,disposed between the two sets of blades, to allow the contactor to havea relatively small width, as required to fit between the meter sprungjaws so that the standard wall box and meter configuration can be used.The terminals 22, 24, of the contactor are connected between a sprungjaw on the meter outlet side and the load connection. This allows themeter stabs to be plugged into the sprung jaws in the conventionalmanner.

A wall box 80 fitted with a disconnect contactor 10 is shown in FIG. 17.The wall box has a meter socket arrange to receive stabs from a standardmeter enclosure. The meter socket includes a plug-in terminals known assprung jaws 82, 83. A supply cable 84 and a load cable 86 and shownentering the wall box and connecting to cable clamps 90 associated withthe meter socket. The supply is a 2 phase supply with phase wires A1, A2and an earth or neutral wire E. The earth wires are shown passing underthe contactor where they are joined together. The supply phase wires A1,A2 connect to sprung jaws 82 in which meter stabs are to be plugged into connect the supply directly to the meter. The meter stabsrepresenting the outlet from the meter plug into sprung jaws 83 whichare isolated from the cable connectors to which the load cable isconnected. Instead, these isolated sprung jaws connect to terminals(here the movable terminals 24) of the contactor 10 and the otherterminals (here the fixed terminals 22) of the contactor are connectedto the cable connectors 92 to which the load phase wires A1′, A2′ areconnected. Thus the supply is fed directly to the meter so that themeter electronics always has power available and the load is suppliedfrom the meter via the disconnect contactor 10, allowing the load to beisolated without turning off the power to the meter.

An advantage of mounting the meter-disconnect contactor outside themeter and inside the wall-box, between the sprung jaws, is that it wouldbe possible to control the switched “disconnect” sprung jaw connection,remotely and independently, of the meter control circuit itself, usingtelemetry or so-called “power-line-carrier” data transmissiontechniques, which are very well developed. It also allows for a simplearrangement to provide an independent remote connect/disconnect facilityusing a simple plug-in type mains meter without a built-in contactor,which is typically smaller and cheaper.

This “integrated” arrangement allows the separation of the meter anddisconnect contactor so that repair or replacement of defective partscan be carried out quickly and easily without replacing additional partswhich are still in good working order. It also allows for a remotelycontrolled “integrated” disconnect contactor in every wall-boxinstallation for remote control of the domestic load connection.

In the description and claims of the present application, each of theverbs “comprise”, “include”, “contain” and “have”, and variationsthereof, are used in an inclusive sense, to specify the presence of thestated item but not to exclude the presence of additional items.

Although the invention is described with reference to one or morepreferred embodiments, it should be appreciated by those skilled in theart that various modifications are possible. Therefore, the scope of theinvention is to be determined by reference to the claims that follow.

1. An electrical contactor comprising: a first terminal connected to apair of fixed contacts on opposite faces of a fixed conductive member; asecond terminal; a pair of movable arms of electrically conductivematerial connected to the second terminal, and carrying movable contactsat an end remote from the connection to the second terminal, the movablearms being arranged in aligned opposition to each other such that theirremote ends are on either side of the fixed conductive member, with themovable contacts aligned with the fixed contacts, and are separated by apredetermined gap over a major portion of their length; and ferrousplates attached on the outer faces of the movable arms, wherein thearrangement of the fixed member and movable arms being such that whenthe contacts are closed, current flowing through the movable arms andthe ferrous plates produces induced magnetic-field attraction forcesbetween the movable arms that urges the movable arms towards each other,thereby increasing the force pressing the movable contacts against thefixed contacts.
 2. The electrical contactor of claim 1, wherein themovable arms are preformed and preloaded so as to bias them towards eachother to engage the fixed contacts with a preset contact pressurekeeping the contacts normally closed in the absence of a forceseparating the movable arms.
 3. The electrical contactor of claim 1,wherein the ferrous plates are attached to the movable arms along theirformed length, whereby when the contacts are closed, higher currentflowing through the movable arms induces magnetic fields in the ferrousplates, generating a magnetic force of attraction urging the contactsclosed.
 4. The electrical contactor of claim 1, comprising an actuatingarrangement including a wedge shaped member disposed between innerinclined surfaces of the movable arms, arranged to separate the movablearms so as to open the contacts, the wedge shaped member being movablefrom a first position in which it separates the movable arms, to asecond position where it allows the arms to move freely towards eachother.
 5. The electrical contactor of claim 4, wherein the actuatingarrangement comprises a movable member that, in a first position is notengaged with the movable arms, to allow the wedge shaped member toseparate the movable arms and in a second position engages with themovable arms to press the movable contacts against the fixed contacts.6. The electrical contactor of claim 5, wherein the movable membercomprises pegs that press against outer inclined surfaces of the movablearms in the second position.
 7. The electrical contactor of claim 5,wherein the outer inclined surfaces of the movable arms are separateflexible tangs integral with the movable arms, which deflect inwardly,to enhance the contact closure and reduce bounce when engaged by themovable member in the second position.
 8. The electrical contactor ofclaim 5, wherein the actuating arrangement comprises an electromagneticactuator coupled to the wedge shaped member and the movable member, theelectromagnetic actuator effecting movement of the wedge shaped memberand the movable member, between the first and second positions.
 9. Theelectrical contactor of claim 8, wherein the wedge and the movablemember are part of a lifter fixed to the electromagnetic actuator. 10.The electrical contactor of claim 1, comprising an actuatingarrangement, the actuating arrangement including an electromagneticactuator, the electromagnetic actuator being released or de-latched tocause the movable contacts to engage the fixed contacts.
 11. Theelectrical contactor of claim 8, in which the electromagnetic actuatoris a magnet-latching solenoid or a permanently-energized, nonmagnet-latching solenoid.
 12. The electrical contactor of claim 1,wherein each movable arm is arranged to carry a substantially equalportion of the total current through the contactor.
 13. The electricalcontactor of claim 1, wherein each movable arm comprises a plurality oflongitudinal sections, each provided with a movable contact at theremote end and arranged to engage with a corresponding fixed contact,the current flow in the arms being substantially equally divided betweenthe sections thereof.
 14. The electrical contactor of claim 1, whereinthe first and second terminals are made of brass.
 15. The electricalcontactor of claim 1, wherein the contactor is a two pole contactorhaving a pair of first and second terminals, a pair of fixed conductivemembers and two pairs of movable arms.
 16. The electrical contactor ofclaim 1, wherein the contacts are arranged in pairs of switch contactsand at least one pair of switch contacts is arranged to close before atleast one other pair of switch contacts, as the contactor changes froman open state to a closed state.